Solve the equation 24p+1+20(4p−1)=32^{4p+1} + 20(4^{p-1}) = 3. Explain why 24p+1+20(4p−1)=k2^{4p+1} + 20(4^{p-1}) = k has no solution if k<−318k < -3\frac{1}{8}.

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Worked solution

4(a)

1. Express the terms using a common base power, such as $4^p$.

24p+1=21⋅(22)2p=2(42p)=2(4p)22^{4p+1} = 2^1 \cdot (2^2)^{2p} = 2(4^{2p}) = 2(4^p)^2
20(4p−1)=20⋅4p4=5(4p)20(4^{p-1}) = 20 \cdot \frac{4^p}{4} = 5(4^p)

2. Substitute $u = 4^p$ to form a quadratic equation.

Let u=4pu = 4^p.
The equation becomes:
2u2+5u=32u^2 + 5u = 3
2u2+5u−3=02u^2 + 5u - 3 = 0

3. Solve the quadratic equation for $u$.

(2u−1)(u+3)=0(2u - 1)(u + 3) = 0
u=12u = \frac{1}{2} or u=−3u = -3

4. Solve for $p$ and reject the invalid solution.

Substitute back u=4pu = 4^p:
4p=124^p = \frac{1}{2} or 4p=−34^p = -3

For 4p=−34^p = -3, there is no real solution since 4p>04^p > 0 for all real pp.

For 4p=124^p = \frac{1}{2}:
22p=2−12^{2p} = 2^{-1}
2p=−12p = -1
p=−12p = -\frac{1}{2}

Answer

p=−12p = -\frac{1}{2}

4(b)

1. Express the equation as a quadratic in terms of $4^p$.

Let x=4px = 4^p.
24p+1+20(4p−1)=k2^{4p+1} + 20(4^{p-1}) = k
2(24p)+20(4p4)=k2(2^{4p}) + 20\left(\frac{4^p}{4}\right) = k
2(42p)+5(4p)=k2(4^{2p}) + 5(4^p) = k
2x2+5x−k=02x^2 + 5x - k = 0

2. Find the discriminant of the quadratic equation.

For the quadratic equation 2x2+5x−k=02x^2 + 5x - k = 0 to have real solutions for xx, its discriminant must be non-negative.
Δ=b2−4ac\Delta = b^2 - 4ac
Δ=52−4(2)(−k)\Delta = 5^2 - 4(2)(-k)
Δ=25+8k\Delta = 25 + 8k

3. Explain the condition for no solution.

If k<−318k < -3\frac{1}{8}, then k<−258k < -\frac{25}{8}.
8k<−25  ⟹  25+8k<08k < -25 \implies 25 + 8k < 0.
Since Δ<0\Delta < 0, the quadratic equation has no real solutions for xx.
Consequently, there are no real solutions for pp.

Answer

The equation reduces to a quadratic in 4p4^p. Its discriminant is negative when k<−318k < -3\frac{1}{8}, meaning there are no real solutions for 4p4^p, and hence no solutions for pp.

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